Ultrafine nickel oxide quantum dots enbedded with few-layer exfoliative graphene for an asymmetric supercapacitor: Enhanced capacitances by alternating voltage

•NiO-dots/Gh has been obtained by alternating voltage approach with one-step.•NiO quantum dots can be uniformly dispersed on the surface of few layers graphene.•The electrochemical mechanism of NiO-dots/Gh structure has been proposed.•The NiO-dots/Gh composite shows high capacity retention.•The NiO-dots/Gh displays high energy density as an asymmetric supercapacitor.

A green and one-step method of electrochemical alternating voltage has been utilized to form NiO quantum dots/graphene flakes (NiO-dots/Gh) for supercapacitor applications. NiO quantum dots (∼3 nm) are uniformly deposited on few-layer graphene surfaces by oxygen functional groups on graphene surface that is naturally utilized to bridge NiO and graphene through Ni–O–C bands, which exhibits outstanding specific capacitance 1181.1 F g−1 at a current density of 2.1 A g−1 and rate behavior 66.2% at 42 A g−1 as NiO dots can be fleetly wired up to current collector through the underlying graphene two-dimensional layers. The NiO-dots/Gh composite is further undertaken in asymmetric supercapacitors with high energy density (27.3 Wh kg−1 at 1562.6 W kg−1).

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